Centralize ssl tests' check for whether we're using LibreSSL.

Right now there's only one caller, so that this is merely
an exercise in shoving code from one module to another,
but there will shortly be another one.  It seems better to
avoid having two copies of this highly-subject-to-change test.

Back-patch to v15, where we first introduced some tests that
don't work with LibreSSL.

Reported-by: Thomas Munro <thomas.munro@gmail.com>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/CA+hUKG+fLqyweHqFSBcErueUVT0vDuSNWui-ySz3+d_APmq7dw@mail.gmail.com
Backpatch-through: 15

src/test/ssl/t/001_ssltests.pl

@@ -35,10 +35,8 @@ sub switch_server_cert
 	$ssl_server->switch_server_cert(@_);
 }
 
-# Determine whether this build uses OpenSSL or LibreSSL. As a heuristic, the
-# HAVE_SSL_CTX_SET_CERT_CB macro isn't defined for LibreSSL. (Nor for OpenSSL
-# 1.0.1, but that's old enough that accommodating it isn't worth the cost.)
-my $libressl = not check_pg_config("#define HAVE_SSL_CTX_SET_CERT_CB 1");
+# Determine whether this build uses OpenSSL or LibreSSL.
+my $libressl = $ssl_server->is_libressl;
 
 #### Some configuration
 

src/test/ssl/t/SSL/Backend/OpenSSL.pm

@@ -26,6 +26,7 @@ package SSL::Backend::OpenSSL;
 
 use strict;
 use warnings;
+use PostgreSQL::Test::Utils;
 use File::Basename;
 use File::Copy;
 
@@ -205,6 +206,25 @@ sub get_library
 	return $self->{_library};
 }
 
+=pod
+
+=item $backend->library_is_libressl()
+
+Detect whether the SSL library is LibreSSL.
+
+=cut
+
+sub library_is_libressl
+{
+	my ($self) = @_;
+
+	# The HAVE_SSL_CTX_SET_CERT_CB macro isn't defined for LibreSSL.
+	# (Nor for OpenSSL 1.0.1, but that's old enough that accommodating it
+	# isn't worth the cost.)
+	# We may eventually need a less-bogus heuristic.
+	return not check_pg_config("#define HAVE_SSL_CTX_SET_CERT_CB 1");
+}
+
 # Internal method for copying a set of files, taking into account wildcards
 sub _copy_files
 {

src/test/ssl/t/SSL/Server.pm

@@ -243,6 +243,23 @@ sub ssl_library
 
 =pod
 
+=item $server->is_libressl()
+
+Detect whether the currently used SSL backend is LibreSSL.
+(Ideally we'd not need this hack, but presently we do.)
+
+=cut
+
+sub is_libressl
+{
+	my $self = shift;
+	my $backend = $self->{backend};
+
+	return $backend->library_is_libressl();
+}
+
+=pod
+
 =item switch_server_cert(params)
 
 Change the configuration to use the given set of certificate, key, ca and